/**
 * Copyright (c) 2010, Todd Ginsberg
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 * 
 *    * Redistributions of source code must retain the above copyright
 *      notice, this list of conditions and the following disclaimer.
 *    * Redistributions in binary form must reproduce the above copyright
 *      notice, this list of conditions and the following disclaimer in the
 *      documentation and/or other materials provided with the distribution.
 *    * Neither the name of Todd Ginsberg, or Gowalla nor the
 *      names of any contributors may be used to endorse or promote products
 *      derived from this software without specific prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT HOLDER> BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 * 
 *  Also, please use this for Good and not Evil.  
 */
package com.ginsberg.gowalla.rate;

import java.util.concurrent.TimeUnit;

/**
 *
 * @author Todd Ginsberg
 */
public class RequestsOverTime implements RateLimiter {

	
	private long[] timestamps; // Treated like a wheel.
	private int timestampPointer = 0; // Pointer to current point on the wheel.
	private long maxWaitTimeMs = 0; // Max amount of time we should ever have to wait.
	
	
	/**
	 * Limit the number of requests over time to the number given.
	 * 
	 * @param maxRequests The number of requests over the time period allowed.
	 * @param periods Number of TimeUnit periods to measure over.
	 * @param units The unit of time to measure over.
	 */
	public RequestsOverTime(final int maxRequests, final int periods, final TimeUnit units) {
		timestamps = new long[maxRequests];
		maxWaitTimeMs = TimeUnit.MILLISECONDS.convert(periods, units);
	}
		
	/**
	 * Limit the number of requests over one second.
	 * 
	 * @param maxRequests The number of requests over the time period allowed.
	 */
	public RequestsOverTime(int maxRequests) {
		this(maxRequests, 1, TimeUnit.SECONDS);
	}
	

	/**
	 * @see com.ginsberg.gowalla.rate.RateLimiter#postRequest()
	 */
	@Override
	public void postRequest() {
		// Nothing to do.
	}

	/**
	 * @see com.ginsberg.gowalla.rate.RateLimiter#preRequest()
	 */
	@Override
	public synchronized boolean preRequest() {
		long waitTime = getWaitTime();
		if(waitTime > 0) {
			try {
				Thread.sleep(waitTime);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}			
		advance();
		return true;
	}
	
	/**
	 * Get the wait time we must endure before unblocking a thread (if any).
	 */
	public long getWaitTime() {
		return Math.max(0, maxWaitTimeMs - (System.currentTimeMillis() - oldestTime()));
	}
	
	/**
	 * Advance the clock with the current time.
	 */
	private void advance() {
		timestamps[timestampPointer] = System.currentTimeMillis();
		timestampPointer++;
		if(timestampPointer == timestamps.length) {
			timestampPointer = 0;
		}
	}
	
	/**
	 * Get the oldest time we know about.
	 */
	private long oldestTime() {
		return timestampPointer == timestamps.length-1 ? timestamps[0] : timestamps[timestampPointer+1];
	}	
}
